1. Field of the Invention
The present invention relates to an optical communications system in which optical signals are transmitted via an optical fiber laid in a transmission section.
2. Related Background of the Invention
In order to reduce the number of repeater stations by elongating each transmission section and to transmit large volumes of information, optical SN ratio (OSNR) is preferably large, in optical communications systems in which optical signals are transmitted via an optical fiber laid in a transmission section. In order to increase OSNR, transmission loss in the optical fiber are preferably small, and the effective area of the optical fiber is preferably large.
Jianjun Yu, et al., ECOC 2008, Th3.E2 (Document 1) disclose results of a transmission experiment that achieved a total transmission capacity of 17 Tbps with 114 Gbps×161 wavelengths by 8PSK modulation, using an optical fiber SMF-28ULL (by Corning) 662 km long and having an average loss of 0.169 dB/km at a wavelength of 1550 nm.
The Corning optical fiber SMF-28ULL catalogue (ver. August 2008) (Document 2) indicates that the central value of the mode field diameter of the Corning optical fiber SMF-28ULL is 10.7 μm at the wavelength of 1550 nm. The effective area of the optical fiber SMF-28ULL is 90 μm2, assuming a Gaussian field distribution.
G. Charlet, et al., ECOC 2008, Th3.E3 (Document 3) disclose results of a transmission experiment in which there was realized a total transmission capacity of 3.2 Tbps with 40 Gbps×81 wavelengths by BPSK modulation, over a transmission distance of 11520 km, with 18 recirculations in a fiber loop 640 km long, using an optical fiber having a loss of 0.184 dB/km and an effective area of 120 μm2.
K. Nagayama, et al., Electronics Letters, 26 Sep. 2002, Vol. 38, No. 20 (Document 4) disclose an optical fiber having, at the wavelength of 1550 nm, a transmission loss of 0.15 dB/km and an effective area of 118 μm2.
U.S. Patent Application aid-Open No. 2008/0279515 (Document 5) discloses an optical fiber having, at the wavelength of 1550 nm, a transmission loss of 0.16 dB/km and an effective area of 208 μm2.
In optical communications systems, optical signals are amplified by way of optical amplifiers, in order to compensate the optical signal loss incurred during the optical signals propagate through the optical fiber that is laid in transmission sections. From the viewpoint of the noise figure (NF) of the optical amplifier, the optical signals in an optical fiber laid in a transmission section are preferably distributed-Raman-amplified.